As one of visible light sources for displaying or lighting, there is a light emitting device using a blue LED or a near-ultraviolet LED based on a gallium nitride-based compound semiconductor such as GaN, GaAlN, InGaN, or InAlGaN. In the light emitting device, white light or other visible light emission can be obtained by using a phosphor material which absorbs a part or all of the emission from the LED as excitation light and converts the wavelength into visible light having a longer wavelength. Particularly, a white LED has been recently widely applied to various indicators, light sources, display devices, and backlights for liquid crystal displays and its use is begun to extend to headlamps for automobiles and general lighting.
Packaging methods of the light emitting device are diversified depending on individual uses and required properties but a “surface-mounting type” capable of surface mounting on a printed wiring board is one of the most mainstream methods. FIG. 20 is a schematic view showing a configuration of a general surface-mounted LED element. A wiring pattern (lead) 32 is formed on the surface of a printed wiring board 31 including a resin or a ceramic material, and an LED element 33 is mounted on the wiring pattern 32 via an adhesive 34 such as a silver paste. An upper electrode of the LED element 33 is connected to another lead 32 with a wire 35 such as a gold wire. In order to protect the wire 35 and the LED element 33, an encapsulating resin is filled to form an encapsulating resin layer 36. In the encapsulating resin layer 36, a powdery phosphor 37 is dispersed. 38 is a reflector, which is provided on the board 31 and becomes a fence for forming the encapsulating resin layer 36 by filling the encapsulating resin as well as has an action to reflect the light emitted from the LED element 33 or the phosphor 37 toward a light extraction direction X side to efficiently utilize the light.
Moreover, as a packaging method of the light emitting device, as shown in FIG. 21, a type where the encapsulating resin layer 39 is formed in a state that only the LED element 33 is covered (chip-coated type) is also in practical use. In this regard, in the chip-coated type in the above FIG. 21, a phosphor (not shown in the figure) is dispersed in the encapsulating resin layer 39 at a high concentration but, in the surface-mounted type in the above FIG. 20, the phosphor 37 is usually dispersed in the encapsulating resin layer 36 at a low concentration.
The following will describe an emission principle of a white LED which is formed by combining a blue LED and a yellow phosphor (generally, a YAG:Ce phosphor). Namely, when electric power is supplied to an LED element from a pair of leads, blue emission takes place. The blue light is transmitted through the encapsulating resin layer but is, on the way, absorbed by the phosphor dispersed in the encapsulating resin layer in a part, whereby the wavelength is converted into yellow color one. As a result, from the semiconductor package, the blue light and the yellow light are radiated in a mixed state but the mixed light is perceived as white color by human eyes. This is an emission principle of the white LED.
Here, when the concentration of the phosphor used is too high, the yellow light becomes too much and a strongly yellowish white color is obtained. On the other hand, when the amount of the phosphor is too small, a bluish white color is obtained. Moreover, even when the phosphor is dispersed in the encapsulating resin at the same concentration, emission color fluctuation occurs owing to various causes such as unevenness in thickness of the encapsulating resin and heterogeneous precipitation of the phosphor during a period until the encapsulating resin is cured. Therefore, it is one problem in the production process of the white LED how to reduce the emission color fluctuation attributable to the arrangement of the phosphor.
Moreover, since the light emitted from the LED element and the phosphor is usually natural light which is radiated to all directions without directivity, the emitted light is radiated not only to the light extraction direction of the package but also to the wiring board side which is an opposite direction, the reflector side, and the like evenly. On this occasion, when a light absorptive material is used in the surface of the wiring board or in the surface of the reflector, the light cannot be efficiently reflected and returned to the light extraction direction. Accordingly, it is devised to impart a reflective function having diffuse reflectivity to the surface of the wiring board or the reflector.
For example, Patent Document 1 proposes a method of mixing a filler for light reflection into an insulating paste for covering the periphery of an LED except for the surface facing to the light emitting direction. Also, there is a description that thermal conductivity of the insulating paste is improved and heat generated from the LED is efficiently radiated to the substrate by mixing the filler. Patent Document 2 proposes an improving method for solving the problem that a resin layer containing a filler for light reflection climbs up to the LED emission surface to lower the emission intensity of the LED in the production step of a light emitting device having a surface-mounted package structure. Patent Document 3 discloses a light emitting device having a structure that all surfaces except for a light exit surface of LED is confined by covering with a resin having a diffuse reflection effect to radiate light only from the light exit surface and having a structure that the light exit surface is covered with a resin containing a phosphor. Patent Document 4 proposes a contrivance that, at the time when the propagating direction of the emitted light from an LED is limited by a resin material having a diffuse reflection effect, a light extraction effect is further improved and luminance is enhanced by setting a forming method thereof to a position lower than a junction position to be provided on the LED.    Patent Document 1: JP-A-2002-270904    Patent Document 2: Japanese Patent No. 3655267    Patent Document 3: JP-A-2005-277227    Patent Document 4: JP-A-2008-199000